This application relates to the art of drive mechanisms and, more particularly, to drive mechanisms for driving a belt past a cylinder around which the belt extends. The invention is particularly applicable for use with belt-type of printing machines and will be particularly described with reference thereto. However, it will be appreciated that the invention has broader aspects and may be used for driving belts in general and is not limited to use for driving belts in belt-type of printing machines.
One example of a belt-type of printing machine is disclosed in U.S. Pat. No. 3,518,940 issued July 7, 1970, to Stroud et al. A continuous belt extends around rolls and is driven through a nip between plate and impression cylinders. The belt carries flexible printing plates which transfer images to a web of paper or the like passing through the nip around the impression cylinder. For halftones and multi-color printing, along with accurate slitting of the printed web, it is necessary to maintain accurate registration between the belt and the belt drive. One driving arrangement for maintaining accurate registration includes belt drive sprockets at opposite ends of the plate cylinder and having sprocket teeth engageable with longitudinally-spaced holes adjacent the longitudinal edges of the belt.
In the earliest drive arrangements of the type described, the sprockets were secured directly to the opposite ends of the plate cylinder and rotatably driven in unison therewith so the plate cylinder also transmitted drive traction to the belt. As a practical matter, it is not possible to machine the plate cylinder to such accurate dimensions that the plate cylinder would drive the belt at precisely the same speed as the drive sprockets. If the plate cylinder drives the belt slightly faster or slower than the drive sprockets, the belt and drive sprockets will slip relative to one another and registration will be lost, or the belt will break. In order to overcome this problem, as disclosed in the aforementioned Stroud patent, the belt drive sprockets are mounted for rotation independently of the plate cylinder. Torque is supplied from the drive sprockets to the plate cylinder through clutches. In addition, the outer surface of the plate cylinder is highly polished so it will slip easily relative to the belt. With this arrangement, it was believed that any tendency for relative movement between the belt and drive sprockets would be overcome by relatively easy slippage between the belt and plate cylinder. However, it has been found that such phenomenon as squeezing of the flexible printing plates between the nip produces a drag on the belt intermediate the drive sprockets and this drag cannot be overcome by relative slippage between the belt and plate cylinder, or between the plate cylinder and drive sprockets. Therefore, it has been found that registration is sometimes lost due to jumping of the sprockets relative to the belt.
It would be desirable to have an arrangement for preventing relative movement between the belt and belt drive sprockets in order to maintain accurate registration of the belt and printing plates carried thereby.